Building on my last post, Acid Bath seem quite compatable with new-metal sensibilities: Grunge/goth influenced sludge. Get him to listen to 'Paegan Terrorism Tactics'. If he likes Slayer, then he should be able to take other thrash bands: Stormtroopers of Death would probably appeal to him, old Metallica's a safe bet too, maybe some of the old thrash giants: Exodus, Kreator and the lesser known but fucking awesome Tankard. Get him into the softer end of the extreme as a stepping stone: MDM bands like the aforementioned Dark Tranquillity, In Flames (He'll like the new album almost certainly, then you can work back), Arch Enemy, Children of Bodom*, Soilwork, maybe At The Gates, Opeth* and Edge of Sanity. Symphonic/Gothic black metal too: Cradle of Filth and Dimmu Borgir are the best known (again, you can start on the latest albums and work back to earlier, superior material) but there's loads of others, often better, that'll appeal of he likes those: Illnath, Hecate Enthroned, Gloomy Grim and Limbonic Art and Cryptic Wintermoon** are some good suggestions, In the Black Metal realm as well you might want to consider proto-BM like Mercyful Fate (and descendant King Diamond), Venom etc. , as well as more punky/rock influenced BM bands such as Carpathian Forest, Abigail and Treblinka***. There are the classics of hard rock and metal of course: Black Sabbath, Iron Maiden, Judas Priest, Alice Cooper, Motorhead, AC/DC and so forth: plus modern descendents like 3 Inches of Blood, Dream Evil, Viking Skull, etc. and Melo-doom/stoner bands like Candlemass and Orange Goblin (candlemass will be good if he's an angsty cunt: Make him listen to 'Solitude' before he burns all his Slipknot CDs: My Dying Bride are in a similiar vein). NIN suggests a liking for Industrial rock...get him into Rammstein, Ministry, KMFDM and Hanzel Und Gretyl. The Decemberists are touch folky aren't they? Folk metal his ass! Finntroll, Skyclad, Waylander, Korpiklaani etc. may be too bizarre for him, or may be his groove. Speaking of groove, there's some pretty accessible groove-based Death Metal: I would suggest Grave, Dismember and Vital Remains personally, as well as some new Death 'n Roll Entombed.
Should be enough to be getting on with...
Also, gentlemen, it takes many a Jack and Coke before I get to the stage I was at last night. I drank about half a large bottle, some vodka and goodness knows how much cider (I remember vaguely swigging from a 2 and a half litre bottle of strongbow like it was lemonade: I think I may have drunk all if it).
*Not technically MDM but we're not here to debate genre semantics.
**Again, not really symph black, but...well, you know the score.
***Obscure as fuck so don't search too hard.